63 Body/Chassis marriage

[Duke W.]

Re: 63 Body/Chassis marriage

The drumming is likely more of an issue with Coupes than Convertibles, but it certainly wouldn't hurt to install some home made "cushions' that were apparently used on early '63s.

Duke

[Con X.]

Re: 63 Body/Chassis marriage

Thanks. Available from Zip, so I'll install them. Can't hurt.

[Mark F.]

Re: 63 Body/Chassis marriage

Maybe I missed it somewhere in this thread, but if resonant frequencies were a problem on '63s (body parts vibrating in harmony with various energies exciting them - causing the annoying "noise"), why were those rubber shims eliminated on the following mid-year production cars ?

Was the problem frequency discovered and then corrected by repositioning of the offending part; use of other (or additional) vibration damping mechanisms; changing its mass or changing its stiffness; whatever ?

[Duke W.]

Re: 63 Body/Chassis marriage

I asked the question, rhetorically, in post #10, but I have no answer. The annoying drumming happened right at about 2000 RPM, but I'm not aware that anyone else has reported such, nor am I aware that there were any changes to the floor pan over the years other than the shallower under-seat recesses, which would not have addressed the stiffness of the luggage shelf at the rear of the luggage compartment which is the part of the floorpan that produced the low frequency noise.

Duke

[Joseph S.]

Re: 63 Body/Chassis marriage

Beginning in 1964, the body was mounted differently. All the body mounts on coupes other than the Radiator support were rubber isolated instead of bolted directly to the frame. This probably solved the issue.

Just a thought.

[Bob J.]

Re: 63 Body/Chassis marriage

I think starting 64 they added a brace to the rear panel of the luggage compartment. Another possibility.

[Leif A.]

Re: 63 Body/Chassis marriage

Beginning in 1964, the body was mounted differently. All the body mounts on coupes other than the Radiator support were rubber isolated instead of bolted directly to the frame. This probably solved the issue.

Just a thought.

This would be my best guess, as well.

[Duke W.]

Re: 63 Body/Chassis marriage

The change to rubber cushion body mounts would have no effect on the natural "drumming frequency" of that large flat section at the rear of the luggage compartment. My mid production SWC (still has the deep seat wells) has three recessed horizontal channels across that area which serve to stiffen it, but without those "shims" it produced the unpleasant "drumming" sound at 2000 RPM in any gear, and it was particularly noticeable when cruising at 2000 in top gear, which with a 3.08 axle is about 50 MPH.

It's possible that area of the floor pan was modified with more stiffening elements to eliminate the vibration thus negating the need for the "shims", like when the tooling was modified in late '63 to the shallow seat wells, but that's just a guess.

As I previously stated I expect the problem was more of an issue with coupes, than convertibles, so has anyone else with a Coupe, noticed this phenomenon?

Duke

[Alan D.]

Re: 63 Body/Chassis marriage

Duke, here is an original 64 (never off frame). Possible mold change - however do not have a 63 kicking around to confirm.
So let's see if somebody posts a picture of early 63.

[Con X.]

Re: 63 Body/Chassis marriage

There is another brace just forward of the one you show and a metal brace vertically.
I'll still install the rubber. Too easy.

[Mark F.]

[in any gear is interesting]

[Con X.]

Re: 63 Body/Chassis marriage

So were you getting an MRI? Lying flat, noisy, trying to distract yourself? That's a lot of thought

[Mark F.]

Re: 63 Body/Chassis marriage

So were you getting an MRI? Lying flat, noisy, trying to distract yourself? That's a lot of thought

Good one, Con !

LOL - You even got "resonance" in your reply (MRI)
nope, I was upright, awake and in a very quiet room - but it was raining...

[Duke W.]

Re: 63 Body/Chassis marriage

We can roughly refer to a C2 Coupe as a Helmholtz resonator. So are small sedans with I-4 engines without balance shafts that typically start "booming" at 4000 RPM driven by the engine's unbalanced second order vertical shaking force.

My '91 MR2 has Helmholtz resonators built into the air clearer assembly and muffler, which yields a pleasant growl, even at low revs, rather than the blatty tractor engine sound that it would have without Toyota's acoustical tricks.

At my first job out of college as a Pontiac production engineer one of the guys told me about a situation with a high revving short geared GTO that had the driveshaft break and come through the floor pan (no injuries) at the proving ground. Near peak engine revs in top gear driveshaft speed approached and then equaled the driveshaft's natural bending frequency and kaboom!

Think of walking across a 2 x 12 at a construction site. If your pace equals the first order natural bending frequency it can bounce up and down enough to throw you off.

The driveshaft in my Cosworth Vega is larger diameter than a 140 Vega to stiffen it, which raises the first order bending frequency. On a good day a 140 Vega probably has a higher top speed in direct drive fourth than overdrive fifth and wouldn't even make 5000 in fourth.

My Cosworth Vega will easily hit 6000 in fifth (in track trim), which means the driveshaft is turning 7500!

Early 240Z models had a habit of breaking crankshafts due to uncontrolled torsional vibration.

World War II aircraft engines had all kinds of torsional vibration issues, especially the V-12 type due to their length, propeller reduction drive, and quill shafts that ran the length of the engine to drive rear mounted accessories like distributors. Back then the engineers (like Duntov) didn't have all the digital simulation tools that can ferret out vibration issues nowadays before anything is built. Back then it was engineering intuition earned through experience that allowed them to attempt to avoid vibration issues in the design and then fix them when testing revealed problems.

Duke

[Mark F.]

Re: 63 Body/Chassis marriage

We can roughly refer to a C2 Coupe as a Helmholtz resonator. Duke

Duke,

Is the "drumming" noise you describe @ 2,000 RPM (in any gear) the same with your windows:
down ?
up ?, or
somewhere in between ?

Do you get the drumming noise at 2k rpm in neutral (IOW nothing spinning aft of the tailpiece) ?

It seems to me that every rotating part aft of the transmission tailpiece is not the energy source of your annoying resonance; particularly if you do get it while in neutral. My reasoning for that statement: The driveshaft, differential, half shafts and tires are all rotating at different speeds in each gear - and not at all in neutral. Essentially, there are 5 different energy states for each of those components when the engine is at 2k rpm.

Unless window position changes it, the only constant I see in your scenario is the engine rotating @ 2,000 RPM, so it must be the source, right ? And the frame must be the conduit to transfer that source to the rear body panels...

I've never noticed it (or at least don't remember it) in mine, but I'm going to check it out next time on a highway...it may be a while, though. Mine's an L68 and now I'm wondering if differences in engine masses may also be at play here, too ?